Regulated mitosis is essential for development and maintenance of cell populations throughout life. Microtubules (MTs) are the main component of the mitotic machinery. MTs are nucleated from the centrosomes, and their plus ends exhibit dynamic instability. The dynamics of MTs are highly regulated by MT associated proteins (MAPs). Proteins of the conserved XMAP215 family are major regulators of MT polymerization that preferentially localize to MT plus ends. During mitosis the XMAP215 family localizes to centrosomes and kinetochores (MT plus ends). Studies of XMAP215 family members have shown that they play an essential role during mitosis, as their depletion leads to short spindles or defects in spindle architecture in S. pombe, C. elegans, Xenopus egg extracts, Drosophila and HeLa cells. Members of the XMAP215 family contain a varying number of N-terminal TOG (tumor over expressed gene) domains that bind to tubulin heterodimers, however what remains to be determined is the structure and functional role of the C-terminal domain. I hypothesize that the Drosophila XMAP215 family member, Minispindles (Msps), uses distinct, conserved determinants to localize to centrosomes and kinetochore plus ends. I hypothesize that Msps activity at each of these locations differentially contributes to mitotic spindle structure. The activity of Msps at each of these locations will affect MT dynamics by promoting nucleation at the centrosomes and increasing MT dynamics at the kinetochore. I will examine these hypotheses by: (1) determining the effect of Msps localization on mitotic spindle architecture, (2) establishing the role of Msps at distinct spindle substructures on MT dynamics (3) elucidate the structure of the Msps C-terminal domain. These aims span the atomic to cellular allowing me to gain a complete understating of how the conserved C-terminal domain of the XMAP215 family member, Msps, affects MT dynamics during mitosis.